Process for producing carbon fiber

ABSTRACT

A process for producing an acrylic carbon fiber comprising the steps of applying an oiling agent comprising as its indispensable component a neopentyl alcohol derivative represented by the following general formula (I) to acrylic fiber, heating the lubricated acrylic fiber in an oxidizing atmosphere to convert the same into oxidized fiber, and heating the oxidized fiber in an inert atmosphere at a higher temperature to carbonize the same: ##STR1## The process can eliminate the defects of acrylic fiber precursor and provide a carbon fiber having excellent mechanical strength characteristics.

This application is a continuation-in-part of application Ser. No.942,650 filed Dec. 17, 1986, now abandoned.

BACKGROUND

The present invention relates to a process for preparing a high qualitycarbon fiber having a high strength by using an acrylic fiber as theprecursor.

Carbon fibers have heretofore been widely used as reinforcing fibers ofvarious composite materials due to their superior mechanical strengthcharacteristics such as excellent specific strength and specificelasticity. In production of a high-grade carbon fiber having anespecially high strength among others, an acrylic fiber is usually usedas the precursor in a generally adopted process comprising the steps ofheating an acrylic fiber in an oxidizing atmosphere at a temperature ofabout 200° to 400° C. to oxidize the same for conversion thereof into anoxidized fiber (flame-retardant fiber), and heating the resulting fiberin an inert atmosphere of nitrogen, helium, argon, or the like at atemperature of about 400° C. or higher to carbonize the same. Thus,acrylic fiber precursor is subjected to a very severe thermal treatmentduring the course of conversion thereof into a carbon fiber.

More specifically, it is believed that acrylic fiber precursor isconverted into a fiber having a thermally stabilized molecular structureas a result of cyclization and crosslinking reactions of polymer chainsconstituting the acrylic fiber in the above-mentioned oxidizing step,while at the same time adherent substances included in the fiber, suchas a oiling agent, are evaporated and thermally decomposed, followed bypolymerization of the resulting thermal decomposition product into atar-like substance. Not only fusion of filaments constituting a fiberbundle among each other but also surface and inside defects of thefilaments themselves are caused by the reactions of the polymer chainsconstituting the fiber and the thermal decomposition of the adherentmatters included in the fiber, such as an oiling agent. Such fusion anddefects drastically deteriorate the physical properties of the carbonfiber. Thus, prevention or suppression of such fusion and defects isvery important in the industrial production of carbon fibers.

Against the above-mentioned problem peculiar to the production of acarbon fiber, a number of proposals have been made of the use of anorganosilicon compound as an oiling agent for an acrylic fiber with aview to suppressing the fusion of filaments among each other in theprocess for production of a carbon fiber, particularly in the oxidizingstep.

However, the use of such an organosilicon oil involves a problem ofworsening of a working environment and causing pollution since theoligomer and terminal molecular chain of the organosilicon oil aredecomposed and evaporated at high temperature and further decomposedinto silicon dioxide in the process for production of a carbon fiber,with the result that a gas exhaust unit is clogged, while dust heaps up.Further, since the organosilicon does not always satisfy the performancerequired of a processing or finishing oil for an acrylic fiber byitself, it is not usually used alone but in combination with otherprocessing or finishing oil. In this respect, however, the effect ofsuppressing fusion of the filaments among each other by theorganosilicon oil is ruined through it depends on the combined oil, orthe defects of the carbon fiber are caused by a tar-like substanceconverted from the combined oil in the above-mentioned oxidizing andcarbonizing steps, thus presenting a problem.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for producinga carbon fiber excellent in mechanical strength characteristics, whichcan eliminates the defects caused in individual filaments constitutingan acrylic fiber as the precursor of the carbon fiber by improving anoil agent.

Another object of the present invention is to provide a process whichcan produce acrylic fiber precursor which can be readily bundled withoutcausing fiber disorder or fluffing and which is free from fusion offilaments in an industrially advantageous manner.

The objects of the present invention can be attained by a processcomprising the steps of applying an oiling agent comprising as theindispensable component a neopentyl alcohol derivative represented bythe following general formula (I) to acrylic fiber, heating thelubricated acrylic fiber in an oxidizing atmosphere to convert the sameinto oxidized fiber bundles, and heating the oxidized fiber in an inertatmosphere of a higher temperature to carbonize the same: ##STR2##[wherein R₁, R₂, and R₃ are each an alkyl group having 1 to 12 carbonatoms, and Y is an alkyl group having 1 to 12 atoms or ##STR3## (whereinR₁, R₂, and R₃ are each an alkyl group having 1 to 12 carbon atoms)].

The use of the above-mentioned specific oiling agent as the one forapplication thereof to the fiber precursor can prevent not only voidsformation inside filaments, heterogeneity of the texture, fusion offilaments among each other, mechanical damages, etc., which are causedin the step of producing a precursor, but also fusion of filaments amongeach other in the step of heating the precursor at high temperature forburning of the same. In this way, a carbon fiber having very excellentmechanical strength characteristics can be obtained.

DETAILED DESCRIPTION OF THE INVENTION

According to the studies of the inventors of the present invention, inaddition to smoothness, antistatic properties, and capability ofbundling required of an oiling agent used in the production of commonsynthetic fibers, the following performance characteristics are veryimportant for an oiling agent used for acrylic fiber in the productionof a carbon fiber:

(1) the oiling agent should not permeate into filaments constituting anacrylic fiber for avoiding fusion of filaments among each other in thestep of producing acrylic fiber, and provide uniform application thereofto the surface of the filaments;

(2) the oiling agent should have a thermal resistance enough to resistheating in the ordinary oxidizing step without forming tar-likesubstance, as well as an excellent releasability; and

(3) in the case of the combined use of the oiling agent with anorganosilicon oil for the purpose of improving the above-mentionedreleasability, the oiling agent should be able to guarantee anindustrially stable operating efficiency without detriment to uniformoil film formation of the organosilicon oil on the surface of filamentsand without reducing the capability of bundling of the acrylic fiber.The above-mentioned performance characteristics can be achieved by usingthe above-mentioned oiling agent according to the present invention.

Although the organosilicon oil is effective in preventing fusion offilaments among each other because of its excellent thermal resistanceand releasability, enormous facilities and expense are needed in thetreatment of an exhaust gas containing a decomposition product of theorganosilicon formed in the burning steps involving oxidation andcarbonization. When fusion occurs prior to application of anorganosilicon oil in the process for production of acrylic fiber, theorganosilicon oil is not effective in eliminating the fusion orpreventing damages to the filament surfaces by peeling of fusedportions, while no effect can be expected in preventing void formationinside the filaments.

Neopentyl alcohol derivatives represented by the formula (I) that can beused in the present invention include neopentyl polyol compounds havingan alkyl group with 1 to 12 carbon atoms in a side chain thereof, suchas trimethylolpropane octanate, pentaerythritol tetralaurate, anddipentaerythritol hexanate.

Further, specific examples include neopentyl glycol dihexanaterepresented by the formula: ##STR4## neopentyl glycol didodecanaterepresented by the formula: ##STR5## trimethylolethane octanaterepresented by the formula: ##STR6## trimethylolpropane trinonanaterepresented by the formula: ##STR7## pentaerythritol tetraoctanaterepresented by the formula: ##STR8## pentaerythritol didodecanatedihexanate represented by the formula: ##STR9## dipentaerythritolhexahexanate represented by the formula: ##STR10## dipentaerythritolhexaoctanate represented by the formula: ##STR11##

When the side chain of the compound as mentioned above is an alkyl groupwith 13 or more carbon atoms, the amount of heating residue isdisadvantageously increased too much.

Although a neopentyl alcohol derivative alone may be used as the oilingagent comprising such a neopentyl alcohol derivative as theindispensable component, a combination thereof with a straight chainorganosilicone represented by the following formula (II) such as anamino-modified polysiloxane, a polyether-modified polysiloxane, or anepoxy-modified polysiloxane is preferably used. Such a combinationserves to highly satisfying properties required of the oiling agent foracrylic fiber as the raw material of a carbon fiber. ##STR12## [whereinR₁ is an alkylene group having 5 or less carbon atoms or an aryl group,x is an integer of 10 to 1000, y is an integer of 1 to 20, A is##STR13## (wherein R₂ is hydrogen or an alkylene group having 5 or lesscarbon atoms, R₃ is hydrogen or an alkylene or aminoalkyl group eachhaving 5 or less carbon atoms, and a is an integer of 1 to 50)].

Examples of the above-mentioned amino-modified siloxane,polyether-modified polysiloxane, and epoxy-modified polysiloxane includeamino-modified polysiloxanes (molecular weight: about 20,000) containingabout 0.7% of primary amino groups and represented by the followingformula: ##STR14## amino-modified polysiloxanes (molecular weight: about8,000) containing primary and secondary amino groups (amino groupcontent: about 0.7%) and represented by the following formula: ##STR15##epoxy-modified polysiloxanes (molecular weight: about 50,000) containingan epoxy content of about 1% and represented by the following formula:##STR16## polyether-modified polysiloxanes (molecular weight: about18,000) containing 10% of polyether groups and represented by thefollowing formula: ##STR17##

Even when such other component is combined, the neopentyl alcoholderivative should be used in an amount of from 20 to 90% by weight,preferably 40 to 80% by weight, based on all components of an oilingagent. The amount of the oiling agent to be applied to acrylic fiber isdesirably about 0.5 to 10% by weight based on the fiber weight. Theamount of the above-mentioned modified organopolysiloxane is desirably10 to 80 wt.% based on the oiling agent.

The acrylic fiber used in the present invention is composed of anacrylic polymer or copolymer containing an acrylonitrile (hereinafterreferred to briefly as "AN") as the main component.

Examples of such an acrylic fiber include not only fibers made of ANhomopolymer, but also fibers respectively made of acrylic copolymer ofat least 90 mol% of AN and less than 10 mol% of a vinyl compoundcopolymerizable with AN, such as acrylic acid, methacrylic acid,itaconic acid, a lower alkyl ester of one of the above-mentioned vinylcompounds, acrylamide, methacrylamide, N-methylolacrylamide, methylvinyl ketone, hydroxyacrylonitrile, acrolein, methacrolein,allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid, or ametallic salt of one of the last three vinyl compounds.

A process for producing acrylic fiber comprises the steps of spinning aspinning solution of an AN polymer as mentioned above in a solventtherefor according to the known wet, dry or dry-jet wet process, andsubjecting the resulting coagulated filaments to the steps includingdrawing, water-washing, and drying to form a fiber. In this process,low-velocity spinning, multistep drawing, etc. are favorably adopted forthe purpose of obtaining a denser fiber.

In this process for producing acrylic fiber, the above-mentioned oilingagent may be used as a processing oil in the steps succeeding waterwashing and drawing, or as a finishing oil after drying. It may also beused after forming fiber. When it is used as the processing oil, it isdesired to comprise a combined oil component selected from among thosehaving antistatic properties, such as nonylphenol, cationic oil, andfatty acid esters, and those capable of providing smoothness, such aswax and adducts of a higher alcohol with ethylene oxide. When it is usedas the finishing oil, it is desired to comprise a mineral oil, an adductof an higher alcohol with ethylene oxide, or a neutral oil for providinga uniform applicability in addition to one having antistatic propertiesand one capable of providing smoothness.

The neopentyl alcohol derivative as the indispensable component of theoiling agent to be used in the process of the present invention has anexcellent performance in respect of thermal resistance, amount ofheating residue, amount of heat generation, and degree of densificationof the resulting fiber, etc. Therefore, it exhibits very excellenteffects of preventing acrylic fiber precursor for production of a carbonfiber from undergoing fusion of filaments among each other in theoxidizing and carbonizing steps and preventing the resulting carbonfiber from involving defects on the surface and inside thereof.Furthermore, since it has properties required of both processing andfinishing oils, the process can be simplified due to non-necessity forthe step of application of any finishing oil when it is used as theprocessing oil.

The following Examples will more specifically illustrate the presentinvention.

In the Examples, the degree of permeation into a fiber and the degree offusion of fiber bundles were examined according to the followingrespective methods.

Degree of Permeation into Fiber (Denseness)

Evaluation is made in terms of heterogeneity attributable to permeationof an oiling agent into the inside of a fiber. More specifically, a rawfilament is dyed with an iodine solution (prepared by diluting 50.76 gof I₂, 10 g of 2,4-dichlorophenol, 90 g of acetic acid, and 100 g ofpotassium iodide (KI) with 11 of water to a predetermined volume). Thehue of the fiber is measured before and after dyeing and a difference inhue therebetween is used to represent the heterogeneity. The larger thehue difference, the larger the heterogeneity up to the inside of thefiber (i.e., the fiber is dyed deep inside of the fiber with I₂).

Degree of Fusion

Filaments of an acrylic fiber bundle are cut to a length of 5 mm anddispersed in a 0.1% aqueous solution of a surface-active agent stirredwith a controlled stirrer, followed by suction filtration and recoveryof filaments. Estimation is made in terms of the number of fused fiberfilaments according to the following rating:

the number of fused filaments of 5 or more: x

the number of fused filaments of 2 or 4: o

the number of fused filaments of 1 or less: ○

EXAMPLES 1 TO 4 AND COMPARATIVE EXAMPLES 1 TO 6

A 19.5% dimethyl sulfoxide (DMSO) solution of an acrylonitrile copolymercomposed of 99.7 mol% of AN and 0.3 mol% of itaconic acid as thespinning solution was spun through a 0.006 mm spinneret into acoagulating bath containing a 55% aqueous DMSO solution as thecoagulating agent. The resulting filaments were sufficiently washed withwarm water to 40° to 65° C., drawn in hot water of 75° to 98° C., anddipped in each one of various oiling agents as listed in Table 1 toapply the same to the filaments, followed by drying to densify the same.Thus, acrylic fiber were prepared.

The acrylic fiber showed a degree of fusion of filaments among eachother as listed in Table 1.

The acrylic fiber was fed to a step of ordinary treatment at 245° C. forimparting a flame resistance to the same and then carbonized in acarbonizing furnace filled with a nitrogen atmosphere maintained at1400° C.

The strand strength and bundle tenacity of the resulting carbon fiberwere measured. The results are shown in Table 2.

It will be understood from Table 1 that acrylic fiber prepared usingoiling agents according to the present invention, namely oiling agentscomprising neopentyl alcohol derivatives and modified polysiloxane,showed a very little permeation of the oiling agent into the inside ofthe filaments thereof and were prevented from fusion of the filaments inthe steps after application of the oiling agent, thus substantiatinguniform application of the oiling agent all over the surfaces of thefilaments.

It will be understood from Table 2 that carbon fibers produced fromacrylic fiber according to the present invention were remarkablysuperior in strand strength and bundle tenacity to those produced usinga higher alcohol or polybutene in Comparative Example as oiling agent.

When more than 80 wt.% of a surface-active agent was blended with anoiling agent according to the present invention as shown in Table 1, thepermeation of the oiling agent into the inside of filaments wereincreased as is apparent from the amount of I₂ (iodine) adsorption andthe strength of the resulting carbon fiber was decreased as shown inTable 2. On the other hand, it will be understood that, when less than10 wt.% of a surface-active agent was blended, the dispersibility of theneopentyl alcohol derivatives/modified polysiloxane the indispensablecomponent of the oiling agent according to the present invention was solowered that uniform application of the oiling agent to the surfaces ofthe fiber filaments was hampered, leading to reduction in the effect ofpreventing the fusion.

Surprisingly, further, it is clearly seen with reference to Tables 1 and2 that in comparison to many of such other oiling agents as representedin the Comparative Examples, the oiling agents according to theinvention incorporating a modified polysiloxane are exceedinglyeffective in suppressing the generation of static electricity and inobtaining a remarkable bundling capability in the process of producingacrylic fiber yarn and in the oxidation step, and can serve to providecarbon fibers having a highly desirable mechanical strength.

Table 3, which is shown below, enters heat resistance values of mainingredients of oiling agents used the in Examples and ComparativeExamples.

                                      TABLE 1                                     __________________________________________________________________________                               Amount of                                                                            Degree of   Degree of                                                  Adhering                                                                             Permeation                                                                          Degree of                                                                           Static                          Kind of Oiling Agent                                                                           Antioxidant                                                                             Oiling Agent                                                                         into  Precursor                                                                           Electricity                                                                         Bundling                  (Compounding Ratio)                                                                            (wt. %)   (wt. %)                                                                              Filament                                                                            Fusion                                                                              Generation                                                                          Capability                __________________________________________________________________________    Example                                                                       dipentaerythritol                                                             hexaluarate (50%)                                                             amino-modified                                                                          nonylphenol                                                         polysiloxane (50%)                                                                      EO adduct                                                           (wt. %)   (wt. %)                                                             95         5     none      3      35    Δ                                                                             slight                                                                              good                      90        10     none      3      36    ○                                                                            slight                                                                              good                      70        30     none      3      38    ⊚                                                                    slight                                                                              good                      50        50     none      3      45    ⊚                                                                    slight                                                                              good                      20        80     none      3      50    ⊚                                                                    slight                                                                              good                      10        90     none      3      59    X     slight                                                                              filament                                                                      disorder                  Example 2                                                                     dipentaerythritol                                                             hexaluarate/amino-                                                            modified  polysilo-                                                                            nonylphenol                                                  xane      EO adduct                                                           70 (wt. %)                                                                              30 (wt. %)                                                          (100/0)*         none      3      35    Δ                                                                             slight                                                                              good                      (95/5)*          none      3      38    ○                                                                            slight                                                                              good                      (90/10)*         none      3      40    ⊚                                                                    slight                                                                              good                      (75/25)*         none      3      45    ⊚                                                                    slight                                                                              good                      (50/50)*         none      3      50    ⊚                                                                    slight                                                                              good                      (0/100)*         none      3      55    Δ                                                                             much  filament                                                                      disorder                  Example 3                                                                     dipentaerythritol                                                             tetraoctanate                                                                 (50%)/amino-                                                                  modified polysilo-                                                                      nonylphenol                                                         xane      EO adduct                                                           (wt. %)   (wt. %)                                                             90        10     none      3      33    ○                                                                            slight                                                                              good                      70        30     none      3      35    ⊚                                                                    slight                                                                              good                      50        50     none      3      43    ⊚                                                                    slight                                                                              good                      20        80     none      3      47    ⊚                                                                    slight                                                                              good                      10        90     none      3      55    Δ                                                                             slight                                                                              filament                                                                      disorder                  Example 4                                                                     dipentaerythritol                                                             tetraoctanate                                                                 (75%)/modified                                                                silicon compo-                                                                          nonylphenol                                                         sition    EO adduct                                                           (wt. %)   (wt. %)                                                             80        20     none      1.5    35    ○                                                                            slight                                                                              good                      80        20     none      3      40    ⊚                                                                    slight                                                                              good                      Comparative Example 1                                                         stearyl alcohol                                                               EO.sub.20 adduct                                                              100 (wt. %)                                                                             none   none      3      60    X     slight                                                                              fluffing                                                                      frequent                  Comparative Example 2                                                         stearyl alcohol                                                               EO.sub.20 adduct (80%)/                                                       amino-modified                                                                          nonylphenol                                                         polysiloxane (20%)                                                                      EO adduct                                                           (wt. %)   (wt. %)                                                             80        20     none      3      55    Δ                                                                             slight                                                                              filament                                                                      disorder                  80        20     none      5      58    ○                                                                            slight                                                                              filament                                                                      disorder                  Comparative Example 3                                                         stearyl alcohol                                                                         none   di(nonylpheny)-                                                                         2.5    52    Δ                                                                             slight                                                                              filament                  EO.sub.20 adduct (50%)/                                                                        dinonylphenyl                      disorder                  amino-modified   phosphite (10%)                                              polysiloxane (40%)                                                            Comparative Example 4                                                         isopentacosanyl                                                                         none   di(nonylpheny)-                                                                         2.6    53    Δ                                                                             slight                                                                              filament                  alcohol EO.sub.20                                                                              dinonylphenyl                      disorder                  aduct (70%)/     phosphite (5%)                                               amino-modified                                                                polysiloxane                                                                  (25%)                                                                         Comparative Example 5                                                         oleyl alcohol                                                                           none   tetrakis[methylene-                                                                     2.5    51    Δ                                                                             slight                                                                              filament                  EO.sub.20 adduct 3(3,5-di-tert-                     disorder                  (70%)/           butyl-4-hydroxy-                                             amino-modified   phenyl)propionate]                                           polysiloxane     methane (5%)                                                 (25%)                                                                         Comparative Example 6                                                         polybutane                                                                              none   none      3.0    40    X     much  fluffing                                                                      frequent                  __________________________________________________________________________     Notes:                                                                        * = compounding ratios (by weight) of dipentaerythritol hexalaurate to        aminomodified polysiloxane                                                    ** = equivalent mixtures of aminomodified silicone, EOmodified silicone       and epoxymodified silicone                                               

                                      TABLE 2                                     __________________________________________________________________________                         Adhering                                                 Composition of       Amount of                                                                            Evaluation                                                                          Strand                                                                              Strength of                           Oiling Agent                                                                             Antitoxidant                                                                            Oiling Agent                                                                         of Tar                                                                              Strength                                                                            Bundle                                (wt. %)    (wt. %)   (wt. %)                                                                              Attachment                                                                          (kg/mm.sup.2)                                                                       (kg)                                  __________________________________________________________________________    Example 1:                                                                    dipentaerythritol                                                                        none      1.0    no                                                hexalaurate (35%)/                                                            aminomodified poly-                                                                      none      2.5    no    455   45                                    siloxane (35%)/                                                               nonylphenol EO                                                                           none      5.0    no                                                adduct (30%)                                                                  Example 2:                                                                    dipentaerythritol                                                             hexalaurate/amino-                                                            modified polysilo-                                                            xane (70%)/nonyl-                                                             phenol EO  adduct                                                             (30%)                                                                         100/0*     none      3      no    380   28                                    95/5*      none      3      no    380   30                                    90/10*     none      3      no    420   40                                    75/20*     none      3      no    460   45                                    50/50*     none      3      no    440   45                                    0/100*     none      3      no    390   45                                    Example 3:                                                                    pentaerythritol                                                                          none      1.0    no    420   40                                    tetraoctanate (40%)/                                                          amino-modified                                                                           none      2.6    no    450   44                                    polysiloxane (30%)/                                                           nonylphenol EO                                                                           none      5.0    no    450   48                                    adduct (30%)                                                                  Example 4:                                                                    pentaerythritol                                                                          none      1.5    no    420   40                                    tetraoctanate (75%)/                                                          modified silicone                                                                        none      3.0    no    450   45                                    composition**                                                                 (25%): 60%                                                                    nonylphenol EO                                                                adduct: (40%)                                                                 Comparative                                                                   Example 1:                                                                    stearyl alcohol                                                                          none      3      much  350   23                                    EO adduct                                                                     Comparative                                                                   Example 2:                                                                    stearyl alcohol                                                                          none      3      fair  410   35                                    EO.sub.20 adduct (80%)/                                                                  none      5      much  400   38                                    amino-modified                                                                polysiloxane (20%)                                                            Comparative                                                                   Example 3:                                                                    stearyl alcohol                                                                          di(nonylphenyl)-                                                                        2.5    fair  410   35                                    EO.sub.20 adduct (50%)/                                                                  dinonylphenyl                                                      amino-modified                                                                           phosphite (10%)                                                    polysiloxane (40%)                                                            Comparative                                                                   Example 4:                                                                    isopentacosanyl                                                                          di(nonylphenyl)-                                                                        2.6    fair  407   31                                    alcohol EO.sub.20 adduct                                                                 dinonylphenyl                                                      (70%)/amino-                                                                             phosphite (5%)                                                     modified polysilo-                                                            xane (25%)                                                                    Comparative                                                                   Example 5:                                                                    oleyl alcohol EO.sub.20                                                                  tetrakis[methylene-                                                                     2.5    fair  40%   33                                    adduct (70%)/amino-                                                                      3(3,5-di-tert-                                                     modified polysilo-                                                                       butyl-4-hydroxy-                                                   xane (25%) phenyl)propionate]                                                            methane (5%)                                                       Comparative                                                                   Example 6:                                                                    polybutane none      3.0    no    300   15                                    __________________________________________________________________________     Notes:                                                                        * = compounding ratios (by weight) of dipentaerythritol hexalaurate and       aminomodified polysiloxane                                                    ** = equivalent mixtures of aminomodified silicone, EOmodified silicone       and epoxymodified silicone                                                    aminomodified silicone: viscosity: 500 cp at 25° C.                    amino content: 1.0%                                                           EOmodified silicone: viscosity: 500 cp                                        EO (ethylene oxide) content: 50%                                              Epoxymodified silicone: viscosity: 1000 cp                                    epoxy content: 1.0%                                                      

                                      TABLE 3                                     __________________________________________________________________________                                   Heat Resis-                                           Oiling Agent                                                                             Antioxidant  tance (°C.)                             __________________________________________________________________________    Example 1                                                                            dipentaerythritol                                                                        none         290                                                   hexalaurate (50%)       (355)                                                 amino-modified                                                                polysiloxane (50%)                                                     Example 3                                                                            pentaerythritol                                                                          none         295                                                   tetraoctanate (50%)     (355)                                                 amino-modified                                                                polysiloxane (50%)                                                     Comparative                                                                          stearyl alcohol EO.sub.20                                                                none         170                                            Example 1                                                                     Comparative                                                                          stearyl alcohol EO.sub.20                                                                none         170                                            Example 2                                                                            (80%)                   (355)                                                 amino-modified                                                                polysiloxane (20%)                                                     Comparative                                                                          stearyl alcohol EO.sub.20                                                                di(nonylphenyl)-                                                                           225                                            Example 3                                                                            (50%)      dinonylphenyl phosphite                                                                    (355)                                                 amino-modified                                                                           (10%)                                                              polysiloxane (40%)                                                                       (10%)                                                       Comparative                                                                          isopentacosanyl                                                                          di(nonylphenyl)                                                                            210                                            Example 4                                                                            alcohol EO.sub.30 (70%)                                                                  dinonylphenyl phosphite                                                                    (355)                                                 amino-modified                                                                           (5%)                                                               polysiloxane (25%)                                                                       (5%)                                                        Comparative                                                                          oleyl alcohol EO.sub.20                                                                  tetrakis[methylene-3(3,5-                                                                  255                                            Example 5                                                                            (70%)      di-tert-butyl-4-hydroxy-                                                                   (355)                                                 amino-modified                                                                           phenyl)propionate]methane                                          polysiloxane (25%)                                                                       (5%)                                                        __________________________________________________________________________

NOTES (1) Method of Evaluation of the Heat Resistance

Of each oiling agent, 10 mg based on the solid component thereof wassampled in a thermobalance, and each sample was heated at a rate ofraising the temperature of 2.5° C./minute to obtain a weight reductioncurve, from which the temperature at which the weight reduction of theoiling agent (solid component) reached 5% was found. The above Tableshows the found temperatures.

(2) The values in parentheses "()" under Heat Resistance in the aboveTable show heat resistance values of the silicon component. When mixtureoiling agents are subjected to heating treatment, there are shownpatterns of reduction in weight proper to respective component oilingagents, that is to say, with respect to the heat resistance, there is nointeraction between or among component oiling agents (providingantioxidants tend to more or less enhance the heat resistance of higheralcohol based oiling agents).

We claim:
 1. A process for producing an acrylic carbon fiber obtained byheating an acrylic fiber in an oxidizing atmosphere to convert the sameinto oxidized fiber and heating said oxidized fiber in an inertatmosphere at a higher temperature to carbonize the same, wherein theacrylic fiber is treated with an oiling agent after it is subjected to awater washing step, said oiling agent comprising (a) from 20 to 90% byweight of a neopentyl alcohol derivative represented by the followinggeneral formula (I): ##STR18## wherein R₁, R₂, and R₃ are each an alkylgroup having 1 to 12 carbon atoms, and Y is an alkyl group having 1 to12 carbon atoms or is ##STR19## wherein R₁, R₂, and R₃ are each an alkylgroup having 1 to 12 carbon atoms, and (b) 10 to 80 wt. % of a modifiedpolysiloxane represented by the following formula (II): ##STR20##wherein R₁ is an alkylene group having 5 or less carbon atoms or an arylgroup, x is an integer of 10 to 1000, y is an integer of 1 to 20, A is##STR21## wherein R₂ is hydrogen or an alkylene group having 5 or lesscarbon atoms, R₃ is hydrogen or an alkylene or aminoalkyl group eachhaving 5 or less carbon atoms, and a is an integer of 1 to
 50. 2. Aprocess for producing an acrylic carbon fiber as claimed in claim 1,wherein the amount of said oiling agent is about 0.5 to 10 wt.% based onthe fiber weight.
 3. A process for producing an acrylic carbon fiber asclaimed in claim 1, wherein said oiling agent further comprises of ananti-static agent, based on the combined weight of the neopentyl alcoholderivative and the modified polysiloxane.